DESCRIPTION (applicant's abstract): In 1972, the suprachiasmatic nucleus (SCN) in the hypothalamus was identified as a timekeeper of mammalian circadian rhythms. The SCN retains its clock properties in vitro, exhibiting a neuronal firing rate rhythm with a period near 24 h in length. The activity rhythms of rodents can be phase advanced in response to a pulse of light given in the late night or to a pulse of darkness given in the day. The SCN electrical activity rhythm in vitro has exhibited similar phase advances in response to application of cAMP analogues during the day and cGMP analogues during the late night. Glutamate is known to be the first message involved in late night-activated phase advances, and downstream communication includes binding to NMDA receptors and activation of cGMP/PKG pathways. Pituitary adenylate cyclase activating peptide (PACAP) is the putative first message mediating daytime-induced phase advances. Both of these neurotransmitters have been localized to the retinohypothalamic tract (RHT), a primary photic input pathway to the SCN. However, conditions permissive for neurotransmitter release from the RHT are not understood. This proposal will test conditions that mediate PACAP and glutamate release from the RHT and will examine how they contribute to SCN phase regulation. Also, it will be determined if RHT stimulation during the daytime leads to activation of a cAMP/PKA-mediated pathway via PACAP. The specific aims are: 1) To develop an RHT electrical stimulation paradigm (via the optic nerve) which causes phase regulation of the SCN, 2) To determine PACAP stimulation leads to activation of the cAMP/PKA pathway and the CREB-family of transcription factors.